Studies on the regulation of cellular metabolism will be carried out at three levels of organization: (1) Cell-free systems which are experimentally manipulated to mimic the physiological range of the energetic steady state of the intact cell have been developed and are in the process of further development in our laboratories. These consist of (a) muscle glycolytic system, which has been developed for studies of the 'Pasteur' effect; (b) liver mitochondrial suspensions incubated with enzymes and substrates which allow metabolic communication between the mitochondrial matrix phase and the remainder of the cell necessary for normal metabolic functions such as gluconeogenesis and fatty acid synthesis; and (c) studies of the control of disposition of carbohydrate (pyruvate) carbon by liver mitochondria at its central metabolic branch-point, i.e., carboxylation of pyruvate to glucose and/or fatty acid precursors, or its oxidative decarboxylation to acetyl-coenzyme A. (2) Isolated intact liver cells will be used for further studies of the control of gluconeogenesis and ureagenesis. (3) A functionally isolated intact hindquarter (muscle) of rats has been developed using a wholly synthetic perfusing medium for studies of metabolic pathways and their regulation in muscle. In particular, relations between amino acid and carbohydrate catabolism and the identification of pathways for, and regulation of, synthesis and depletion of citrate cycle intermediates in muscle will be studied.